1. Field of the Invention
This invention relates to a method and apparatus for monitoring and realigning telecommunication antennas with the emitter markers of satellites.
2. History of the Related Art
Earth telecommunication stations communicate with geostationary satellites via antennas of variable diameter (up to 32 meters). To ensure good communication, these antennas must be sighted on emitter markers installed on the and which and permanently emit a constant signal.
Being given that a satellite drifts slowly in the course of time and that an antenna is subjected to the wind, it is necessary to provide an automatic resighting system for controlling the elevation and azimuth adjustment shafts of the antenna.
Devices capable of ensuring resighting of the antennas are known.
A first solution consists in producing a system which generates an electrical signal proportional to the deviation between the axis of incidence of the satellite signal and the axis of sight of the antenna. This signal is applied independently to the azimuth shaft and the elevation shaft. This system is known as a "Monopulse system". The function of resighting is performed by a proportional control completed by an appropriate filtering.
This solution is technically without reproach. The error of sighting is zero, on average, and the effect of the gusts of wind is substantially divided by 2. However, the cost of angular deviation measurement is high and, as furthermore present-day technology makes it possible to reduce the diameter of the antennas (and therefore their price), it seems that this technique is not practical in the long run, for economical reasons.
A second solution consists in industrially producing various algorithms for detecting the optimum position. It is generally a question of step-tracking. The process alternates steps of displacements and of stops during which the signal received is analyzed in order best to define the following displacement. A slightly different method may also be used, employing the derivative with respect to time of the signal received. The antenna advancing along an axis at constant speed, the signal received is derived by a digital filtering. As long as the derivative is positive, advance continues in the same direction. When the derivative becomes negative, the movement stops and the antenna starts again in opposite direction before scanning the other axis. This process forms the subject matter of U.S. Pat. No. 4,358,767.
However, the present solutions raise some problems.
It has been seen that the antenna was subjected to the wind which may have a considerable effect on a light structure of large diameter taking into account the precisions demanded (some tens of millidegrees). The decisions of antenna adjustment must be effected despite a random misalignment due to the wind. Furthermore,--and this is the most delicate problem--the marker signal may be very disturbed when passing through the atmosphere, i.e. its theoretically constant value varies in fact considerably in time. In the extreme cases, the amplitude of the noise may be 100 times greater than that of the useful signal. The algorithms of maximum tracking by successive displacements or adjustments are then questionable since they cannot discern whether the variation of the registered signal is due to the displacement of the antenna or to the atmospheric disturbance.
The improvements forming the subject matter of the present invention aim at overcoming these drawbacks and at allowing a process of resighting to be developed which responds better than heretofore to the desiderata of the technique.